Adaptive cruise control (ACC), sometimes also called autonomous or radar cruise control, is a cruise control system for vehicles that automatically adjusts the vehicle speed to maintain a safe distance from preceding vehicles. Control of the distance is often imposed based on sensor information from on-board sensors only. The on-board sensors may be for example a radar or a laser sensor, allowing the vehicle to slow down when approaching another, preceding vehicle and accelerate again to a pre-set speed when the preceding vehicle accelerates.
Thus the ACC will attempt to keep a certain distance to the preceding vehicle in front, upon activation of the system. The distance may be depending on an adjustable time gap that the driver has selected, for example between 1-5 seconds. It is often recommended to keep a 3 second time gap to the preceding vehicle, regardless of the speed.
For a better performance/security of the ACC system when driving in hilly terrain, an adaptation of the time slot is sometimes made, depending on the slope of the road. In downhill the distance may be set a little longer, in uphill the distance may be set a little shorter. The estimation of the slope is made by a slope sensor in the following vehicle.
However, when driving in a hilly terrain, the preceding vehicle, or target vehicle, will arrive to a slope before the following vehicle. Thus the target vehicle will be driving uphill, often with reduced speed while the following vehicle still is driving on uninclined or differently inclined ground, and consequently the following vehicle has to brake in order to keep the time gap. Some seconds later, the following vehicle arrives to the uphill and its speed is affected by gravity. Thus, the following vehicle has to accelerate in order to keep the time gap to the target vehicle.
In this example, kinetic energy is firstly braked away, only to be required some seconds later in order to keep the time gap to the target vehicle.
The reverse problem may appear in the opposite situation when the target vehicle starts driving downhill and increases speed due to gravity, while the following vehicle may be driving on uninclined or differently inclined ground, or uphill and thus the following vehicle has to accelerate in order to keep the time gap, and then to start braking some seconds later.
Energy is thereby unnecessarily wasted. Also, the brakes are unnecessarily used, which may lead to need for early replacement due to wear.
The described problems will appear on any vehicle using an ACC system. However, the problems will increase with weight, both for the target vehicle and the following vehicle. In particular heavy vehicles like trucks, busses etc. are affected.
However, another problem is that the target vehicle and the following vehicle may be affected differently when driving uphill, for example. In case the target vehicle is an unloaded vehicle, it may hardly be affected at all by the uphill, while a heavily loaded following vehicle may have to accelerate heavily in order to keep up the pace.
Yet another problem may be that some weather or road conditions may require a prolonged time gap to the target vehicle, such as slippery road due to icy conditions, diesel fuel spills on the road, wet leaves on the road etc.
As these described scenarios, and similar variants of them, will lead to increased fuel consumption, it is desirable to find a solution.